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The basic breathing rhythm is a reflex action under the control of the nervous system The region of the brain controlling this basic rhythm is the medulla oblongata The medulla contains a breathing centre consisting of two groups of nerve cells, called the inspiratory and expiratory centres Nerves arising from these centres innervate (make contact with) the intercostal muscles and the diaphragm The thoracic nerves innervate the intercostal muscles The phrenic nerves innervate the diaphragm Control of Rhythmic Breathing

At the height of an inspiration, the alveoli are inflated and stretched, thus stimulating stretch receptors in their walls A pattern of nerve impulses travels along the vagus nerve to the respiratory centres leading to inhibition of the inspiratory centre and stimulation of the expiratory centre Impulses travelling along the thoracic and phrenic nerves from the expiratory centre lead to relaxation of the diaphragm and intercostal muscles The alveoli deflate and stretch receptors are no longer stimulated EXPIRATION FOLLOWS Expiratory centre Inspiratory centre stimulate inhibit A pattern of nerve impulses travels along the vagus nerve to the respiratory centres leading to stimulation of the inspiratory centre and inhibition of the expiratory centre thoracic nerves phrenic nerves stimulate inhibit Impulses travelling along the thoracic and phrenic nerves from the inspiratory centre lead to contraction of the diaphragm and intercostal muscles INSPIRATION FOLLOWS phrenic nerves thoracic nerves

The purpose of the breathing rhythm is to ventilate the lungs to allow delivery of oxygen to the alveoli, and elimination of the waste gas carbon dioxide from the alveoli As a consequence of gas exchange at the alveoli, there are differences between the composition of inhaled and exhaled air Another factor that contributes to the differences found between inspired and expired air, is the dead space content The dead space is the region of the respiratory tract where NO gas exchange takes place Gas exchange only takes place across the thin walls of the alveoli trachea bronchi bronchioles The air filling the trachea, bronchi and bronchioles is unavailable for gas exchange and is said to occupy dead space A healthy adult, at rest, inspires approximately 600 cm 3 of air of which about 150 cm 3 fills the airways The volume of air actually reaching the alveoli is thus about 450 cm 3 As the air passages are never completely emptied of air, there is only a partial replacement of air in the lungs Composition of Inspired and Expired Air

The table below can be used to explain what happens to air as it enters and leaves the respiratory system It is important to realise that the lungs can never be completely emptied of air; even following a forced expiration, air remains within the alveoli and this amount of air is called the residual volume Composition of Inspired and Expired Air The Relative Composition (% by Volume) of Inspired, Expired & Alveolar Air Gas Inspired air % Expired air % Alveolar air % Oxygen Carbon dioxide Water vapour Nitrogen

Inspired air contains approximately 21% by volume of oxygen gas; as this fresh air is drawn into the alveoli, it mixes with air already present (the residual volume) The residual volume dilutes the fresh air, such that the oxygen content falls to about 67% of that in the atmosphere The oxygen content of alveolar air now falls even further as oxygen diffuses from the alveoli in to the blood along its concentration gradient The Relative Composition (% by Volume) of Inspired, Expired & Alveolar Air Gas Inspired air % Expired air % Alveolar air % Oxygen Carbon dioxide Water vapour Nitrogen Composition of Inspired and Expired Air

The carbon dioxide content of alveolar air increases significantly as gas exchange proceeds and carbon dioxide diffuses from the blood into the alveoli The oxygen content of expired air is higher than that in the alveoli and is intermediate in value between that atmospheric air and alveolar air This is explained by the fact that expired air from the alveoli mixes with the dead space air whose oxygen content is the same as that of the atmosphere The Relative Composition (% by Volume) of Inspired, Expired & Alveolar Air Gas Inspired air % Expired air % Alveolar air % Oxygen Carbon dioxide Water vapour Nitrogen Composition of Inspired and Expired Air

The percent by volume of carbon dioxide in expired air is less than that of alveolar air Again, this is explained by the fact that expired air from the alveoli mixes with the dead space air containing very low levels of carbon dioxide The Relative Composition (% by Volume) of Inspired, Expired & Alveolar Air Gas Inspired air % Expired air % Alveolar air % Oxygen Carbon dioxide Water vapour Nitrogen Composition of Inspired and Expired Air

The Relative Composition (% by Volume) of Inspired, Expired & Alveolar Air Gas Inspired air % Expired air % Alveolar air % Oxygen Carbon dioxide Water vapour Nitrogen The water vapour content of expired air is significantly higher than that of inspired air; as air is breathed into the alveoli, water from the lining of the alveoli evaporates into the alveolar air such that expired air is greater in volume than inspired air Nitrogen gas is neither used or produced by the body and actual amounts of nitrogen in inspired an expired air do not change The slightly larger volume of expired air means that nitrogen forms part of a larger volume during expiration and so its percent by volume decreases Composition of Inspired and Expired Air

The volumes of air inspired and expired in different circumstances, can be measured using an instrument called a spirometer The spirometer consists of a large tank of water, onto which rests a large, and very light perspex lid A nose clip is placed on the subject to prevent any air being lost from the system through the nose tank of water light, perspex lid A counterweight on the edge of the lid is used to balance the box, so that its edges just rest under the surface of the water counter -weight A series of pipes lead from from the air under the lid of the box to the mouthpiece mouthpiece A set of valves ensures that inspired and expired air travel along different pipes valves The subject breathes air into and out of the space under the lid via the mouthpiece Expired air is passed over soda lime to absorb CO 2 gas thus preventing the subject from inspiring increasing amounts of this gas Volume changes associated with breathing are recorded with a pen from the lid onto a rotating drum (kymograph) kymograph soda lime Measuring Lung Volumes

As the subject breathes in through the mouthpiece, the lid moves down

As the subject breathes in through the mouthpiece, the lid moves down As the subject breathes out through the mouthpiece, the lid moves up

As the subject breathes in through the mouthpiece, the lid moves down As the subject breathes out through the mouthpiece, the lid moves up

time This graph shows the results of a spirometer recording; it is customary to display spirometer traces upside down with inspiration curves moving upwards and expiration curves moving downwards Kymograph Recording of Lung Volumes

time The volume of air breathed in an out during one ventilation cycle, or breath, is called the TIDAL VOLUME tidal volume The tidal volume is found to vary from 0.4 to 0.6 dm 3 in healthy subjects; following strenuous exercise it can rise to around 3.0 dm 3

time The air we normally breathe in and out, does not represent our full capacity for inspiration or for expiration tidal volume If a subject is asked to take as deep a breath as possible, i.e. force an inspiration, we obtain a trace of the INSPIRATORY CAPACITY inspiratory capacity In order to achieve their inspiratory capacity, subjects must continue to inhale after a normal inspiration The extra amount of air that can be inhaled following a normal inspiration is called the INSPIRATORY RESERVE VOLUME inspiratory reserve volume

time Subjects can also force an expiration, although the extra volume of air that can be expired is less than obtained in a forced inspiration tidal volume inspiratory capacity As with inspiration, we can obtain traces for EXPIRATORY CAPACITY and EXPIRATORY RESERVE VOLUME expiratory capacity expiratory reserve volume inspiratory reserve volume

time If we add together the inspiratory and expiratory capacities, that is the maximum volume of air that can be exchanged during one breath in and out, we have a measure of the VITAL CAPACITY tidal volume inspiratory capacity The average vital capacities are about 5.5 dm 3 for a male and 4.25 dm 3 for a female expiratory capacity expiratory reserve volume inspiratory reserve volume vital capacity

time The lungs cannot be completely emptied and a certain volume of air always remains in the lungs even following a forced expiration tidal volume inspiratory capacity This measurement cannot be made using a spirometer and requires more sophisticated techniques; values of about 1.5 dm 3 are reported for residual volumes expiratory capacity expiratory reserve volume inspiratory reserve volume vital capacity This is called the RESIDUAL VOLUME residual volume

time The TOTAL LUNG CAPACITY is therefore the sum of of the vital capacity and the residual volume tidal volume inspiratory capacity expiratory capacity expiratory reserve volume inspiratory reserve volume vital capacity residual volume Spirometer tracings can be used to determine a variety of physiological measurements such as metabolic rate, breathing rate and oxygen consumption total lung capacity

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